A typical subsea wellhead assembly has a high pressure wellhead housing supported in a lower pressure wellhead housing and secured to casing that extends into the well. One or more casing hangers land, i.e. are supported by the wellhead housing, and the casing hangers being located at the upper end of a string of casing that extends into the well to a deeper depth. A string of tubing extends through the casing for production fluids. A xmas or production tree is mounted to the upper end of the wellhead housing for controlling the well fluid. The production tree is typically a large, heavy assembly, having a number of valves and controls mounted thereon for controlling the passage of well fluid through the production tree.
One type of production tree, sometimes known as a “conventional tree” has two bores, one of which is a production bore and the other bore is the tubing annulus access bore. In this type of wellhead assembly, the tubing hanger is supported by the wellhead housing and the tubing hanger has two passages through it; one passage being the production passage and the other passage being an annulus passage which communicates with the tubing annulus surrounding the tubing. Access to the tubing annulus is necessary to circulate fluids down the production tubing and up through the tubing annulus or vice versa to either kill the well or circulate heavy fluid during completion. After the tubing hanger is installed and before the drilling riser is removed for installation of the tree, the downhole safety valve is closed and plugs are temporarily placed in the passages of tubing hanger; this is known as well suspension.
The conventional tree has isolation tubes that stab into engagement with passages in the tubing hanger when the tree lands in the wellhead housing. This type of tree is normally run on a completion riser that has two strings of conduit and this is known as a dual bore completion riser. In such a completion riser, one string extends from the production passage of the tree to the surface vessel, whilst the other string extends from the tubing annulus passage in the tree to the surface vessel.
To assemble and run such a dual bore completion riser is very time-consuming. In addition, drilling vessels may not have such a completion riser available, requiring one to be supplied on a rental basis and, furthermore, in deeper waters it is often technically difficult to configure such a dual bore riser.
With such conventional tubing hanger types, the tubing hanger is installed before the tree is landed on the wellhead housing and tubing is typically run on a small diameter riser through the drilling riser and blow-out preventer (BOP). Before the drilling riser is disconnected from the wellhead housing, a plug is installed in the tubing hanger as a safety barrier. This plug is normally lowered on a wireline through the small diameter riser. After the tree is installed the plug is then removed through the riser that was used to install the tree.
This sequence of events requires that a mobile offshore drilling unit (MODU) type of vessel is necessary to affect well desuspension because conduits must be established between the vessel and the production tree through which plugs may subsequently be pulled. It is desirable to be able to permit a well to be desuspended without the need to establish a dual bore riser to surface and thereby permit non-MODU type vessels to conduct xmas tree installation operations and desuspension operations.
Published international application WO 03/067017 (ABB Vetco Gray) discloses a hydraulic ram which is used to retrieve a plug from a tubing hanger. Although this arrangement allows the plug to be retrieved through a wellhead, it also requires a separate package to be run, established with the xmas tree, operated and retrieved, thus incurring substantial additional operational costs and risk.